Many processes for modifying the characteristics of the polymers comprising introducing polar groups, e.g., anionic groups (such as a sulfonic acid group and a carboxylic acid group) and cationic groups (such as a quaternary ammonium group) as constitutional units of polymers, have been known.
Polyurethane resins have excellent flexibilities or mechanical properties and have been widely used for various applications. In the introduction of a polar group into a polyurethane resin, a diol component having a polar group is usually employed as a raw material. In the introduction of a cationic group, a variety of diol components having quaternary ammonium groups are known. On the other hand, in the introduction of an anionic group into a polyurethane resin, a diol component having a sulfonic acid group or a carboxylic acid group (and a salt thereof) is used.
In the polyurethane resin, particularly, regarding an aqueous solution or an aqueous dispersion of polyurethane resin, processes for producing an aqueous solution or an aqueous dispersion of polyurethane resin has been proposed, for example, a process comprising dispersing a polyurethane resin in water by using a surfactant (a dispersing agent) and a process comprising using a hydrophilic polyol (such as a polyethylene glycol) as a polyol component to give a self-dispersed urethane resin. From the viewpoints of the performances or the water resistance of the resin, introducing a hydrophilic group such as an anionic group or a cationic group into a polyurethane resin is advantageous to a process for producing an aqueous solution or an aqueous dispersion of polyurethane resin.
In the introduction of a polar group such as a hydrophilic group into a urethane resin, using a raw material monomer or oligomer having such a polar group in synthesis of a polymer has been generally known. The introduction of a polar group into a polyurethane resin usually employs a diol component having a polar group [e.g., a sulfonic acid group or a carboxylic acid group (and a salt thereof)] as a raw material.
Regarding the introduction of a polar group into a polyurethane resin, an introduction of a carboxyl group into a polyurethane resin is disclosed. For example, a process for producing a water-soluble polyurethane resin disclosed in Japanese Examined Patent Application Publication No. 15874/1978 (JP-53-15874B, Patent Document 1) comprises allowing a polyisocyanate or a urethane prepolymer having an isocyanate group at a terminal thereof to react with a polyhydroxy compound obtained by a reaction of a polycarboxylic acid anhydride with diethanolamine or hydroxylamine and having at least one carboxyl group and allowing the reaction product to react with an organic amine, ammonia or an inorganic base. However, in this process, the introduction of a carboxyl group into a hard segment of the polyurethane in the chain extension increases the aggregation of the polyurethane, and a sufficient water-solubility or water dispersibility is not obtained. Moreover, a high-boiling polar solvent (e.g., dimethylformamide) has a higher compatibility with a polyurethane than a low-boiling polar solvent and is used for the reaction in the production of the polyurethane. The high-boiling polar solvent is difficult to remove after hydrophilization of the polyurethane and consequently remains in the aqueous resin. In addition, a carboxyl group is a weak anion and has an insufficient effect on the modification of the polymer.
Further, an introduction of a carboxylate group and/or a sulfonate group is disclosed in Japanese Patent Application Laid-Open Publication No. 69882/1988 (JP-63-69882A, Patent Document 2). The process comprises using a diol component or a diaminosulfonic acid such as sodium N-(2-aminoethyl)-2-aminoethanesulfonate or sodium dimethylolpropionate as a chain extender in a urethane synthesis to introduce a carboxylate group and/or a sulfonate group. However, even in this introduction, a sulfonic acid (a salt thereof) group is introduced into a hard segment of the polyurethane, which increases the aggregation and cannot give the sufficient water-solubility or water dispersibility.
Furthermore, Japanese Patent Application Laid-Open Publication No. 354742/2001 (JP-2001-354742A, Patent Document 3) suggests an aqueous solution and an aqueous dispersion of an aqueous polyurethane resin containing a salt of N,N-bis(2-hydroxyethyl)-2-aminoethanesulfonic acid quaternary ammonium through a urethane bond in a molecular chain thereof. However, in particular, a diol component having a high content of a sulfonate group, e.g., N,N-bis(2-hydroxyethyl)-2-aminoethanesulfonic acid salt, is hardly soluble or insoluble in a low-boiling organic solvent used in a production of the polyurethane and difficult to use. Accordingly, a composition or molecular weight of an oligomer is limited and a molecular design of a polyurethane resin is prevented. Incidentally, a combination use of the low-boiling organic solvent and a high-boiling polar solvent (such as dimethylformamide or N-methylpyrrolidone) as a reaction solvent in the reaction makes it difficult to remove the high-boiling solvent used in the reaction after hydrophilization, and the high-boiling organic solvent consequently remains in the aqueous resin.
In addition, the following introductions of a sulfonic acid group have been known: a process comprises once dissolving a N,N-bis(2-hydroxyethyl)-2-aminoethanesulfonic acid tetraethylammonium salt in N,N-dimethylacetamide, and allowing the salt to react with an isocyanate to introduce a sulfonic acid group into a polyurethane resin [Japanese Patent Application Laid-Open Publication No. 48858/2001 (JP-2001-48858A, Patent Document 4)] and a process comprises transesterifying sodium 5-sulfodimethyl isophthalate by using a solvent (e.g., ethyleneglycol, dimethylacetoamide, and manganese acetate), removing methanol, and allowing the reaction product to react with an isocyanate to introduce a sulfonic acid group into a polyurethane resin [Japanese Patent Application Laid-Open Publication No. 15500/2005 (JP-2005-15500A, Patent Document 5)]. However, in these processes, the content ratio of the sulfonic acid group is inevitably decreased and the compositions or molecular weights of oligomers are restricted, which prevents the molecular design of a polyurethane resin.
[Patent Document 1] JP-53-15874B (Claims, and page 3, the fifth column, lines 24 to 27)
[Patent Document 2] JP-63-69882A (Claims, and page 5, lower right column, line 11 to page 6, upper left column, line 3)
[Patent Document 3] JP-2001-354742A (Claim 1, Examples)
[Patent Document 4] JP-2001-48858A (Claim 1, Examples)
[Patent Document 5] JP-2005-15500A (Claim 4, Examples)